How to use it
Enter how much you'd put into I bonds and how long you expect to keep them. The tool compares two bonds bought with the same money: one issued in October 2026 and one issued in November 2026. It values both on the same calendar date, so the comparison is fair, and it subtracts the 3-month penalty when that date is less than 5 years in.
Two numbers aren't known yet: the new fixed rate Treasury will set for November bonds, and September's inflation. Pick what you want to assume for each. The tool then shows which bond comes out ahead, by how much, and the break-even fixed rate: the lowest new fixed rate at which waiting would pay off.
Why the fixed rate decides it
An I bond's rate has two parts. The fixed rate is set when you buy and stays the same for 30 years. The inflation rate changes every 6 months for every I bond, old and new. So the inflation part mostly washes out between an October bond and a November bond: each gets the same announcements, just one month apart.
What doesn't wash out is the fixed rate. An October bond keeps 0.90% for 30 years. A November bond gets whatever Treasury announces for November, and there is no published formula for it. If it stays at 0.90%, buying in October comes out slightly ahead because October's first 6 months earn today's 4.26%. If Treasury raises it enough, waiting wins, and the longer you hold, the less of a raise it takes.
The catch: the new fixed rate is announced around Nov 1, after the last October buying day. This tool can't tell you what Treasury will do, and it doesn't guess. It tells you the line, so you can decide how you feel about the odds.
The October deadline
Your bond's issue date is the first day of the month TreasuryDirect receives your money. Funds must arrive before midnight Eastern time. Oct 31, 2026 is a Saturday, so the last business day is Fri, Oct 30. TreasuryDirect warns that purchases at the end of the month may get the next month's issue date, so aim earlier than the last day. An October bond earns interest for all of October, even if you buy it on the 30th.
How the Nov 1 rate is set
The inflation part comes from a formula in Treasury's rules (31 CFR 359.11): the change in CPI-U, not seasonally adjusted, from March to September 2026, rounded to 0.01%. March was 330.213. The latest index, August, is 334.980, already 1.44% above March. In a typical September (the median change from 2014 to 2025), the rate would be about 1.63%, close to today's 1.67%. Past Septembers range from 1.29% (like 2015) to 1.98% (like 2017). A September index of 335.712 or more keeps it at 1.67%.
The composite rate is then fixed + 2 × inflation + fixed × inflation, rounded to 0.01%. Today's 4.26% is 0.90% fixed and 1.67% inflation. BLS publishes September's CPI on Wed, Oct 14, 2026 at 8:30 a.m. ET; after that this page shows the inflation part calculated from BLS data instead of a forecast. See every formula and the rounding rules.
Example: $10,000, fixed rate unchanged
With $10,000.00, the Nov 1 inflation part at 1.63% and the new fixed rate at 0.90%, here is each bond's value on the same date, and the fixed rate at which waiting would have caught up.
| Hold | Buy by Oct 30 | Wait | Waiting wins from |
|---|---|---|---|
| 1 yearNov 1, 2027 | $10,352.00+$40.00 ahead | $10,312.00 | 1.36% |
| 2 yearsNov 1, 2028 | $10,788.00+$40.00 ahead | $10,748.00 | 1.08% |
| 5 yearsNov 1, 2031 | $12,336.00+$48.00 ahead | $12,288.00 | 0.97% |
| 10 yearsNov 1, 2036 | $15,164.00+$60.00 ahead | $15,104.00 | 0.93% |
| 30 yearsOct 1, 2056 | $34,504.00+$140.00 ahead | $34,364.00 | 0.92% |
These are estimates. Every later rate uses the same inflation assumption, and the values follow TreasuryDirect's published method to the cent. Taxes aren't included: I bond interest is subject to federal income tax but not state or local income tax.
Already own an I bond? See exactly what it's worth, with the 3-month penalty, the way TreasuryDirect calculates it. To buy, use TreasuryDirect (opens in a new tab).
Updated . Rates since 1998: 57 announcements.